Lakes are an important surface water resource in South Dakota. Many lakes
are used as water supplies for human and livestock use. They also support
boating, fishing, swimming and other recreational activities. Water quality
in Pactola and Deerfield reservoirs is considered very good with high transparency
and relatively low primary productivity (German 1997). Factors that limit
nutrient availability and primary productivity in Black Hills Reservoirs,
however, are not well understood. In Pactola Reservoir, the upper Castle Creek
drainage flows through natural bog iron deposits. While considered a local
water quality problem, high iron content in downstream water may be important
in reducing phosphorus availability in Pactola Reservoir. Under aerobic conditions,
iron is the most important agent immobilizing dissolved, reactive phosphorus
in aquatic environments. When the hypolimnion of lakes and reservoirs become
anoxic phosphorus is released into solution contributing to increased reservoir
nutrient concentrations and increased productivity. Hence, iron concentrations
and dissolved oxygen levels could play an important role in limiting phosphorus
availability and maintaining water quality in Black Hills reservoirs. Sheridan
and Stockade reservoirs may be particularly susceptible to 'internal' nutrient
loading because the hypolimnion in these systems can become anoxic in summer
months (German 1997). Moreover, these reservoirs are located in different
watersheds than Pactola and Deerfield reservoirs where nutrient availability
may not be regulated by high iron content in the water.

Biological communities of lakes and reservoirs can also have an important
influence on primary productivity and resulting water quality (Carpenter et
al. 1985). Food web structure, for example, can play an important role in
regulating the abundance and composition of phytoplankton. While fish abundance
and composition are well documented, the role of fish planktivory on zooplankton
biomass in Black Hills reservoirs has not been documented.

4)Determine if lake trophic state has degraded in these lakes since the
1991-1995 study.

Sediment cores will be collected in each reservoir seasonally from October
2000 to December 2001. Iron and total phosphorus will be measured in surficial
sediments to determine iron-to-phosphorus ratios for each reservoir (Jensen
et al. 1992). Iron:phosphorus ratios will then be compared to phosphorus concentrations
in the water column to derive empirical relationships for predicting nutrient
availability in each reservoir (Jensen et al. 1992).

In-lake water quality samples will be collected with a Van Dorn type water
sampler within six days of mid-month in June, July and August in 2001. Water
samples will also be collected from several discreet locations in each reservoir
to determine spatial variability of total phosphorus concentrations. Vertical
profiles of several parameters will also be collected at the deepest point
in each reservoir using a YSI DataSonde. Surface and bottom water samples
will be collected and analyzed for soluble reactive phosphorus (SRP) and chlorophyll-a
biomass. Other parameters that will be collected seasonally include; vertical
profiles of temperature, dissolved oxygen, pH, conductivity, total dissolved
solids, turbidity, ammonium, nitrate, and secchi depth.

To complete the third objective and to quantify size-structure and composition
of planktonic zooplankton, samples will be collected with a Wisconsin net
at seasonal intervals from October 2000 to December 2001. The ratio of small-to-large
cladocerans (e.g. Bosmina:Daphnia) will be used to assess the relative magnitude
of 'top-down' food web interactions in each reservoir. Data on fish composition
and abundance will be compared to zooplankton size-structure and biomass.
Estimates of chlorophyll-a and zooplankton biomass will be used to examine
relationships with Fe:P ratios and phosphorus availability in each reservoir.

The fourth objective will be completed by comparing data collected in 2001
with data collected in the 91-95 study.